DOI: 10.1002/chem.201501151
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& Heterogeneous Catalysis
Design of a Heterogeneous Catalyst Based on Cellulose Nanocrystals for Cyclopropanation: Synthesis and Solid-State NMR Characterization Jiquan Liu ,[a] Andreas Plog,[b] Pedro Groszewicz,[a] Li Zhao,[a] Yeping Xu,[a] Hergen Breitzke,[a] Annegret Stark,[c] Rudolf Hoffmann,[d] Torsten Gutmann,*[a] Kai Zhang ,*[e, f] and Gerd Buntkowsky*[a] Abstract: Heterogeneous dirhodium(II) catalysts based on environmentally benign and biocompatible cellulose nanocrystals (CNC-Rh2) as support material were obtained by ligand exchange between carboxyl groups on the CNC surface and Rh2(OOCCF3)4, as was confirmed by solid-state 19F and 13C NMR spectroscopy. On average, two CF3COO¢ groups are replaced during ligand exchange, which is consistent with quantitative analysis by a combination of 19F NMR
Introduction Homogeneous transition metal catalysts often exhibit excellent performance in the industrial production of organic compounds, but show disadvantages regarding recovery and recyclability, especially if biotoxicity is an issue or legislation on metal contamination of pharmaceutical ingredients is tight.[1] Dirhodium(II) catalysts having a paddle-wheel structure with catalytic sites at the axial positions (see Supporting Information, Scheme S1) have been employed as an efficient tool in organic synthesis to produce intermediates, for example, in drug synthesis by forming C¢C bonds through carbenoid transformations, or in C¢H insertion and cyclopropanation.[1d, 2] Among the reactions catalyzed by dirhodium(II) complexes, cyclopropanation is an efficient way to build the cyclopropyl moiety in certain pharmaceuticals, such as the antidepressant [a] J. Liu ,+ P. Groszewicz, L. Zhao, Dr. Y. Xu, Dr. H. Breitzke, Dr. T. Gutmann, Prof. Dr. G. Buntkowsky Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry Technische Universitt Darmstadt Alarich-Weiss-Strasse 8, 64287 Darmstadt (Germany) E-mail: [email protected] [email protected]
[b] A. Plog Center of Smart Interfaces, Technische Universitt Darmstadt Alarich-Weiss-Strasse 10, 64287 Darmstadt (Germany) [c] Prof. Dr. A. Stark College of Agriculture, Engineering and Science, School of Engineering University of KwaZulu-Natal, Howard College Campus, Durban (South Africa)
spectroscopy and thermogravimetry. CNC-Rh2 catalysts performed well in a model cyclopropanation reaction, in spite of the low dirhodium(II) content on the CNC surface (0.23 mmol g¢1). The immobilization through covalent bonding combined with the separate locations of binding positions and active sites of CNC-Rh2 guarantees a high stability against leaching and allows the recovery and reuse of the catalyst during the cyclopropanation reaction.
milnacipran.[3] Furthermore, they are of interest for enantioand diastereoselective synthesis.[3b] However, unsatisfactory recovery and recycling of dirhodium(II) complexes limit their applications.[1d] Various possibilities for the immobilization of chiral and achiral dirhodium(II) complexes have been discussed previously, such as anchoring the complex in the pores of mesoporous silica[4] or on the side chains of a polymer.[5] Recently, some of us have shown that a dirhodium(II) complex can be grafted on bifunctionalized SBA-15 materials through carboxyl and amino groups, and the structure was determined by 13C and 15N solidstate NMR techniques.[6] In all these works, covalent bonds are achieved by ligand exchange and/or axial coordination.[4, 5f,g, 6] However, for most of these catalysts only moderate activity is observed, probably due to the mass-transfer limitations in porous materials.[7] [d] Dr. R. Hoffmann Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry Technische Universitt Darmstadt Alarich-Weiss-Strasse 12, 64287 Darmstadt (Germany) [e] Prof. Dr. K. Zhang + Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science Technische Universitt Darmstadt Alarich-Weiss-Strasse 8, 64287 Darmstadt (Germany) [f] Prof. Dr. K. Zhang + Wood Technology and Wood Chemistry, Georg-August-Universitt Gçttingen Bìsgenweg 4, 37077 Gçttingen (Germany) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501151.
Chem. Eur. J. 2015, 21, 12414 – 12420
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Full Paper Crystalline nanocelluloses including cellulose nanocrystals (CNC) are a class of biocompatible materials that are formed during the oxidation of microcrystalline cellulose. During the oxidation process carboxyl groups are generated on the surface. These materials have attracted much attention in the last few years,[8] and they have been utilized as templates for chiral structures,[9] as reinforcing components,[10] and recently as supports for photoswitchable molecules such as rhodamine spiroamide.[11] A built-in advantage of CNC is the high specific surface area due to its nanoscale diameter and length.[8b, 12] As proposed by Quignard and Choplin,[13] nanocellulose including CNC has excellent properties as support for catalysts such as CuI,[7b] CuO,[14] Au,[15] AuAg alloy,[16] and Pd nanoparticles.[17] Despite this application potential of CNC as support for nanocatalysts, it has, to the best of our knowledge, not been used for grafting of metal complexes to construct well-recyclable immobilized homogeneous catalysts. Such novel catalyst systems open up new scope in fine-chemical synthesis and the pharmaceutical industry. Using abundant sustainable nanomaterials and minimizing potential heavy-metal pollution is clearly the method of choice for environmentally benign production pathways. Accordingly, the present work describes an efficient way to anchor a dirhodium(II) complex on the surface of CNC through covalent bonds with surface carboxyl groups. The structure of the resulting CNC-Rh2 and details of ligand exchange between the dirhodium(II) precursor and the CNC carrier were determined by solid-state 19F and 13C NMR spectroscopy. The catalytic performance of CNC-Rh2 was investigated in the cyclopropanation of styrene with ethyl diazoacetate (EDA) as a typical model reaction, and the stability against leaching and reutilization of this type of catalyst were studied.
Experimental Section Materials Microcrystalline cellulose (MCC, crystal size of 50 mm), ethyl diazoacetate (EDA), and styrene were purchased from Sigma-Aldrich (Steinheim, Germany). 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) and rhodium trifluoroacetate dimer (Rh2(OOCCF3)4) were obtained from Acros (Geel, Belgium). The dialysis membrane with an approximate molecular weight cutoff of 1000 Da was purchased from Spectrum Laboratories Inc. (Rancho Dominquez, USA). All chemicals were employed without further purification.
Preparation of CNC
Synthesis of CNC-Rh2 The heterogeneous CNC-Rh2 catalyst was obtained by ligand exchange.[6, 19] In a typical procedure, Rh2(OOCCF3)4 (0.1 g, 1.0 equiv) and the acidic form of CNC (CNC-COOH, 1.2 equiv based on the number of carboxyl groups determined by titration, see Supporting Information) in 150 mL of EtOAc was charged into a 250 mL round-bottomed flask. This flask was fitted with a Soxhlet extractor in which an oven-dried mixture of K2CO3 and 4 æ molecular sieves (1 g:1 g) was placed in a cellulose filter tube. To optimize the synthesis, the exchange was performed at 110 8C for 0.5, 2.5, and 4.5 d, respectively. After separation of CNC-Rh2 by centrifugation, the solid was washed three times with both EtOAc and CH2Cl2. The obtained catalysts were stored in CH2Cl2 in a freezer at ¢25 8C. Sample names are abbreviated as CNC-Rh2-0.5d, CNC-Rh2-2.5d, and CNC-Rh2-4.5d, according to the exchange time. A sample containing physisorbed dirhodium(II) complex on CNC (CNC-Rh2-ad) was prepared by simply mixing CNC and Rh2(OOCCF3)4 in EtOAc, followed by centrifugation and washing with EtOAc and CH2Cl2.
Cyclopropanation, leaching, and reusability The cyclopropanation of styrene with EDA was used to evaluate the performance of the heterogeneous CNC-Rh2 catalyst at room temperature.[2c, 4b, 6] Styrene (0.52 g) was transferred to a 25 mL glass bottle containing a solution of 0.035 g of the catalyst in 13.3 g of CH2Cl2, and the mixture was treated with ultrasound for 15 min. The reaction started when EDA (0.057 g in 6.65 g CH2Cl2) was added at room temperature. The reaction mixture was filtered and the clear solution was analyzed by GC (Agilent Technologies 7820A, equipped with an HP-5 (30 m Õ 0.32 mm Õ 0.25 mm) column and FID detector). Leaching was evaluated by a hot filtration test,[20] which was performed simply by centrifugal separation (6000 rpm) at room temperature. Typically, CNC-Rh2-4.5d was removed after 15 min reaction, and CNC-Rh2-ad after 60 min. The recyclability test on CNC-Rh2-4.5d was carried out like the regular catalytic reaction. The catalyst and product were separated by centrifugation (6000 rpm, room temperature) at the end of each reaction run, and the catalyst was reused for the next cycle.
Characterization
CNC was prepared according to literature procedures.[18] Typically, 5 g of MCC was swollen in 50 mL of water for 96 h prior to oxidation. Then, MCC was separated by centrifugation and redispersed in 250 mL of water with vigorous stirring. Thereafter, TEMPO (0.0781 g, 0.5 mmol) and NaBr (0.5144 g, 5 mmol) were added to the cellulose suspension and the mixture was stirred for a further 30 min. An aqueous solution of NaClO (10.89 mL, 14 wt %, 25 mmol) was progressively added to the mixture at pH 10. After addition of the NaClO, the pH value was further maintained at pH 10 by adding 1 m aqueous NaOH. After 15 min of ultrasonic treatment of the reaction mixture at ambient temperature, an additional 10.89 mL of aqueous NaClO (14 wt %) was progressively added to the mixture, and the oxidation was repeated at pH 10. Chem. Eur. J. 2015, 21, 12414 – 12420
After adjustment of the pH value to 7.5 by using 1 m aqueous HCl, the oxidized cellulose was isolated by centrifugation (15 min, 4500 rpm), redispersed in water, and ultrasonicated at room temperature for 1 h. Finally, the suspension was dialyzed until the conductivity reached about 1 mS cm¢1 and its volume was adjusted to 200 mL.
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Solid-state MAS NMR spectroscopy: Solid-state 19F MAS NMR spectra were recorded at 564.2 MHz on a Bruker Avance III spectrometer with a broadband double-resonance 1.3 mm probe. All spectra were typically accumulated with a recycle delay of 3 s and a spinning rate of 60 kHz. The 19F chemical shifts were referenced to trifluoroacetic acid at ¢76.5 ppm. The fluorine content was determined by quantitative 19F MAS NMR (see Supporting Information for details). 13C CP MAS NMR was carried out with a broadband double-resonance 4 mm probe at 150.81 MHz. 13C CP MAS spectra were recorded with a recycle delay of 3 s, 3 ms contact time, and a spinning rate of 6 kHz. The 13C chemical shifts were referenced to TMS at 0 ppm.
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Full Paper Thermogravimetry (TG): TG measurements were carried out with TG209F1-Iris from Netzsch (Selb, Germany). Each sample was analyzed under an oxygen atmosphere in the range of 30–800 8C with a heating rate of 10 8C min¢1 on an alumina crucible. Atomic force microscopy: AFM images were obtained on a scanning force microscope (MFP-3D, Asylum research, Santa Barbara, USA) with a cantilever (Budget Sensors, Sofia, Bulgaria, spring constant kc 48 N m¢1, resonance frequency 190 kHz) in tapping mode. The samples were deposited from suspensions on a silicon wafer and dried overnight. The programs IGOR Pro 6.22A (WaveMetrics Inc., Lake Portland, USA) and Gwyddion (Free Software Foundation, Inc., Boston, USA) were used to guide and analyze the AFM measurements, respectively.
Results and Discussion Immobilization of the dirhodium(II) complex on CNC CNC with carboxyl groups on the surface was prepared by TEMPO-mediated oxidation of microcrystalline cellulose (Figure 1 a).[17, 20] During oxidation, the amorphous regions inside the cellulose fibers are cleaved, which leads to the formation of CNC. The primary hydroxyl groups at the C6 position on the surface of cellulose fibers are transformed first into aldehyde groups and further into carboxyl groups during the oxidation
(Figure 1 a).[18] The contents of carboxyl and aldehyde groups were determined to be 1.101 and 0.053 mmol g¢1, respectively, by conductive titration (Supporting Information, Figure S1).[12, 22] Similar to the literature,[23] the resulting CNC exhibits a diameter of approximately 6 nm and a length of around 200 nm according to AFM measurements (Figure 1 b–d). In the next step, the dirhodium(II) moieties were anchored on the surface of CNC by ligand exchange between Rh2(OOCCF3)4 and carboxyl groups (Figure 1 e). The trifluoroacetic acid byproduct of the exchange reaction was evaporated, condensed, and then neutralized with K2CO3 during Soxhlet extraction.[19] The resulting CNC-Rh2 catalyst still maintained the dimensions of CNC with an average diameter of about 8 nm and a length of about 200 nm (Figure 1 f–h). The presence of the rhodium was confirmed by energy-dispersive X-ray spectroscopy (EDX) analysis (Supporting Information, Figure S2), and the amount of dirhodium(II) moieties on CNC was determined by TG (Figure 2). According to the literature, Rh2O3 is formed when dirhodium(II) species are heated at 500 8C in TG analysis under an atmosphere of air[24] and is stable up to 1100 8C.[25] Therefore, the amount of dirhodium(II) moieties in each sample was calculated from the amount of residue at 800 8C after TG analysis. The fraction of dirhodium(II) units in CNC-Rh2-4.5d was estimated to be 0.23 mmol Rh2 per gram of the material. For CNC-Rh2-0.5d and CNC-Rh2-2.5d values of 0.19
Figure 1. a) Schematic illustration of the synthesis of CNC by TEMPO-mediated oxidation. b), c) Representative AFM images of CNCs at different magnifications: scale bar of 500 nm in (b) and 200 nm in (c). d) Height profile of the CNC in (c). e) Schematic illustration of the preparation of CNC-Rh2. f), g) Representative AFM images of CNC-Rh2-4.5d at different magnifications: scale bar of 500 nm in (f) and 200 nm in (g). h) Height profile of the CNC in (g). Chem. Eur. J. 2015, 21, 12414 – 12420
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Figure 2. TG curves and amount of residue after destruction in oxygen atmosphere (30–800 8C) for pristine CNC (d) and CNC-Rh2-4.5d (c).
Figure 3. Deconvoluted solid-state 19F MAS NMR spectra of CNC-Rh2-4.5d (a) and the physical mixture of CNC and neat Rh2(OOCCF3)4 (b).
and 0.22 mmol Rh2 per gram of the material were found (Supporting Information, Figure S3 a), and for the sample CNC-Rh2ad containing physisorbed catalyst the amount of Rh2 was below the detection minimum of the TG analysis. (Supporting Information, Figure S3 b) Since the preparation of heterogeneous CNC-Rh2 was carried out in analogy to the route for generation of homogeneous dirhodium(II) complexes,[19] it was assumed that the replacement of each carboxylate group in the dirhodium(II) complex occurs stepwise, as shown by kinetic studies in the literature.[26] Additionally, Doyle et al.[2c] pointed out that pure Rh2(HNOCCH3)4 is only obtained from Rh2(OOCCH3)4 by trapping the acetic acid, that is, removal of this byproduct can drive the exchange reaction to completion. In our work, the ligand-exchange reaction takes place on the surface of CNC, and the gradual removal of trifluoroacetic acid indeed promoted the ligand exchange. 19
F Solid-state NMR characterization
Since the ligand-exchange process occurs stepwise,[26] the remaining trifluoroacetate on the CNC-Rh2 may provide information on the dirhodium(II) units and the structure of this catalyst after ligand exchange. Thus, solid-state 19F MAS NMR measurements were performed. The signals in the 19F spectrum of CNC-Rh2-4.5d (Figure 3 a), which are located in the region of those of neat Rh2(OOCCF3)4, indicate the presence of the dirhodium(II) units bearing trifluoroacetate on CNC. The spectrum of CNC-Rh2-4.5d shows a main peak at about ¢76 ppm, which is related to the presence of trifluoroacetate ligands coordinated to dirhodium(II) units on the surface of CNC. In addition, a less intense component shifted by ¢3 ppm from the main peak is observed. This signal seems to be attributable to the proximity of water molecules, which influence the chemical environment of the trifluoroacetate ligand and thus the 19F chemical shift. This assumption is based on the observed influence of humidity on the solid-state 19F NMR spectrum of the neat Rh2(OOCCF3)4 complex (see Supporting Information, Figure S4). To support this hypothesis, a dry physical mixture of CNC and neat Rh2(OOCCF3)4 (1.5 wt %, ca. 0.23 mmol g¢1) was invesChem. Eur. J. 2015, 21, 12414 – 12420
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tigated. Only one peak at ca. ¢76 ppm was observed in the 19F spectrum (Figure 3 b), which implies that the extra peak in the immobilized sample is attributable to the presence of water, which is formed by neutralization of the trapped CF3COOH with K2CO3 during catalyst preparation. Furthermore, the amount of fluorine on CNC was determined quantitatively by 19F NMR spectroscopy and was found to be 1.32 0.23 mmol g¢1 for CNC-Rh2-4.5d (for details, see Supporting Information). Taking the Rh2 fraction (0.23 mmol g¢1) on CNC-Rh2-4.5d into consideration, the CF3COO¢/Rh2 ratio is 1.91 0.33, which is close to a factor of two. This means that, on average, two CF3COO¢ groups remained in the catalyst after ligand exchange. This observation is in good agreement with a kinetic study on ligand exchange between trifluoroacetic acid and Rh2(OOCCH3)4.[26] It showed that 1) ligand exchange proceeded stepwise, 2) the second substitution step occurred trans to the first one, and 3) the third and fourth substitution had much lower rate constants than the first two steps; this led to the assumption of transsubstituted CNC-Rh2 (Figure 1 e). 13
C Solid-state NMR characterization
In Figure 4, solid-state 13C CP MAS NMR spectroscopy was employed to analyze the chemical environment of CNC-Rh2 in comparison with pristine CNC, especially for carboxyl groups in the neutral (CNC ~ COONa) or the acidic form (CNC ~ COOH). The spectra in Figure 4 b show strong signals that belong to the cellulose domain (50–110 ppm) and carboxyl carbon atoms (165–190 ppm). The signals at 104 and 65 ppm are attributed to C1 and C6 in the anhydroglucose units of cellulose, respectively.[11, 27] The signals between 74 and 72 ppm are ascribed to C3/C2/C5, and the signals at 88 and 84 ppm are related to crystalline and amorphous C4, respectively.[28] The carbonyl regions of the spectra are similar to those previously reported for the CNC–rhodamine system.[11] For CNCs with carboxyl groups in their acidic (CNC ~ COOH) and salt (CNC ~ COONa) forms, signals at 171 and 174 ppm, respectively, are observed. A signal at 174 ppm is also observed in the spectrum of the
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Figure 5. a) Schematic representation of the catalytic model cyclopropanation reaction.[4b] b) Catalytic performance of the heterogeneous CNC-Rh24.5d catalyst: total yield and percentages of the cis and trans isomers as functions of the reaction time. c) Leaching test: reaction in the presence of CNC-Rh2-4.5d (1) and after removal of CNC-Rh2-4.5d (1’), as well as in the presence of CNC-Rh2-ad (2) and after removal of CNC-Rh2-ad (2’).
Figure 4. a) Schematic illustration of the synthesis of CNC in its COOH and COONa forms. b) Solid-state 13C CP MAS NMR spectra of CNC ~ COOH (top), CNC ~ COONa (middle), and CNC-Rh2-4.5d (bottom). Signals marked with asterisks are spinning side bands.
CNC-Rh2-4.5d sample, which clearly indicates CNC ~ COONa. In addition, a very weak signal with low signal-to-noise ratio around 188 ppm is observed. Similar chemical shifts were observed for the carboxyl carbon atoms of the linkers in the supported Rh2(OOCCH3)4.[6] Thus, this signal is an indication of carboxyl groups coordinated to rhodium on the surface of the CNC. Activity tests The cyclopropanation of styrene with EDA was employed as model reaction to investigate the performance of the heterogeneous CNC-Rh2 catalyst (Figure 5 a). To determine synthesis conditions under which the heterogeneous CNC-Rh2 catalyst performs best, the catalytic reaction was performed with equal amounts of the CNC-Rh2-0.5d, CNC-Rh2-2.5d, and CNC-Rh2-4.5d immobilized catalysts (Supporting Information, Figure S6). From the GC results, the trans isomer is slightly favored for most of the heterogeneous catalysts, but the cis:trans ratio is always close to 1:1 (Supporting Information, Figure S8). This is similar to the results observed for cyclopropanation of styrene with homogeneous Rh2(OOCCF3)4.[2c, 29] CNC-Rh2-4.5d (Figure 5 b) exhibited the highest yield of about 75–80 % in 180 min at room temperature. For comparison, CNC-Rh2-ad with physisorbed Rh2(OOCCF3)4 on the CNC surface was investigated (Figure 5 c). It exhibited the lowest yield of only 7 %, which is not surprising, because the amount of rhodium in the sample is below the detection minimum of the TG analysis (see Supporting Information, Figure S3 b). This result demonstrates that the contribution of physisorbed Rh2(OOCCF3)4 on CNC to the overall yield is negligible in comparison to its covalently immobilized counterpart. Chem. Eur. J. 2015, 21, 12414 – 12420
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The catalytic activities at room temperature of the new heterogeneous catalyst and its homogeneous counterpart Rh2(OOCCF3)4 were compared. The concentration of the homogeneous catalyst was chosen according to the Rh content of CNC-Rh2-4.5d. The homogeneous catalyst reached a yield of 84 % in 1 min, which then increased to 95 % after 1 h (see Supporting Information, Figure S7 a). The initial rate constants for the heterogeneous and homogeneous catalysts were approximated by assuming a first-order reaction. From these fits (see Supporting Information, Figure S7), 0.044 min¢1 was calculated for the heterogeneous catalyst, as opposed to 2.13 min¢1 for the homogeneous catalyst. This shows that, although the immobilized catalyst exhibits excellent performance in the catalytic cyclopropanation under mild conditions, its activity seems to be limited by mass transfer and loss of mobility, because the catalytic centers are confined on the surface of the CNC support, and transport processes are the rate-determining factor. Leaching test In the leaching test performed at room temperature on CNCRh2-ad (Figure 5 c), the yield of product in the filtrate solution stayed nearly constant following removal of CNC-Rh2-ad after 60 min, which indicates that the small amount of physisorbed catalyst is stable against leaching and thus the catalytic activity in this sample stems from catalyst molecules that interact with the CNC. We assume that this robustness against leaching results from the factor that CNC stabilizes the Rh2(OOCCF3)4 on its surface through physical interactions. In the case of CNC-Rh2-4.5d, the reaction did not take place after the catalyst was removed by centrifugation for 15 min (Figure 5 c), which confirms that significant leaching of active sites does not occur. According to the results of solid-state NMR spectroscopy, the covalent bonds are formed between the carboxyl group of CNC and dirhodium units, whereby the carboxyl groups coordinate with dirhodium moieties at equa-
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Full Paper torial positions (see Figure 1 e). The catalytic sites of the dirhodium catalyst are at the axial position, where a rhodium carbenoid is formed (see Supporting Information, Scheme S1).[1d, 2b] Therefore, the separation of catalytic sites (axial direction) and binding sites (equatorial position) seems to prevent release of the dirhodium(II) moieties from the CNC surface. The recyclability of CNC-Rh2-4.5d was investigated under similar conditions to the catalytic test (Figure 6). The obtained yields in the first three runs stayed almost at the same level, with a slight decrease of only 5 % demonstrating its excellent recyclability in the cyclopropanation reaction.
mogeneous catalysts, employing dirhodium(II) as a real-world example. The potential application in constructing sustainable immobilized catalysts is not limited to dirhodium(II), and various candidates seem to be feasible for grafting on the CNC surface by using our methodology for heterogenization of homogenous catalysts.
Acknowledgements The authors thank the Hessian excellence initiative LOEWE – research cluster SOFT CONTROL and Deutsche Forschungsgemeinschaft (DFG) under contract Bu 911-12-2 for the financial support. Keywords: cellulose · cyclization · nanoparticles · rhodium · supported catalysts
Figure 6. Recyclability test of CNC-Rh2-4.5d in the cyclopropanation between styrene and EDA. The yield reached after each run is plotted.
Conclusion A heterogeneous CNC-based dirhodium(II) catalyst was prepared by ligand exchange with sustainable cellulose nanocrystals as biocompatible support. This heterogeneous catalyst exhibits good catalytic performance even at low dirhodium(II) loadings (0.23 mmol g¢1). The structure of this catalyst was investigated by solid-state NMR spectroscopy. 13C CP MAS NMR spectra confirmed a chemical bond between the dirhodium(II) moiety and carboxyl groups on CNC, and 19F MAS NMR spectroscopy indicated the existence of trifluoroacetate groups on CNC-Rh2. On the basis of quantitative 19F NMR and TG analysis, an average of two trifluoroacetate ligands are replaced by COOH groups on the CNC surface. The content of the dirhodium(II) units on the CNC carrier increases to some extent at longer exchange reaction times. CNC-Rh2-4.5d shows the highest concentration of catalytic sites among the prepared catalysts. Its superior performance in the model cyclopropanation reaction indicates that the majority of the Rh sites are available and play an active role in the reaction. The leaching test showed no release of catalytic sites from the CNC support during the cyclopropanation reaction, and the recyclability test revealed excellent recovery and reusability of this heterogenized catalyst. The lower rate constant obtained for the immobilized catalyst compared to the homogeneous dirhodium(II) catalyst is typical for a change from homogeneous to heterogeneous catalysis, for which transport properties are often the ratedetermining factor. However, the lower overall rate constants are a small trade-off for its ease of separation from the reaction medium, its recyclability, and its possible application in continuous reaction processes. Thus, we have shown that CNCs are an excellent support material for the heterogenization of hoChem. Eur. J. 2015, 21, 12414 – 12420
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Received: March 24, 2015 Published online on July 14, 2015
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& Heterogeneous Catalysis
Design of a Heterogeneous Catalyst Based on Cellulose Nanocrystals for Cyclopropanation: Synthesis and Solid-State NMR Characterization Jiquan Liu ,[a] Andreas Plog,[b] Pedro Groszewicz,[a] Li Zhao,[a] Yeping Xu,[a] Hergen Breitzke,[a] Annegret Stark,[c] Rudolf Hoffmann,[d] Torsten Gutmann,*[a] Kai Zhang ,*[e, f] and Gerd Buntkowsky*[a] Abstract: Heterogeneous dirhodium(II) catalysts based on environmentally benign and biocompatible cellulose nanocrystals (CNC-Rh2) as support material were obtained by ligand exchange between carboxyl groups on the CNC surface and Rh2(OOCCF3)4, as was confirmed by solid-state 19F and 13C NMR spectroscopy. On average, two CF3COO¢ groups are replaced during ligand exchange, which is consistent with quantitative analysis by a combination of 19F NMR
Introduction Homogeneous transition metal catalysts often exhibit excellent performance in the industrial production of organic compounds, but show disadvantages regarding recovery and recyclability, especially if biotoxicity is an issue or legislation on metal contamination of pharmaceutical ingredients is tight.[1] Dirhodium(II) catalysts having a paddle-wheel structure with catalytic sites at the axial positions (see Supporting Information, Scheme S1) have been employed as an efficient tool in organic synthesis to produce intermediates, for example, in drug synthesis by forming C¢C bonds through carbenoid transformations, or in C¢H insertion and cyclopropanation.[1d, 2] Among the reactions catalyzed by dirhodium(II) complexes, cyclopropanation is an efficient way to build the cyclopropyl moiety in certain pharmaceuticals, such as the antidepressant [a] J. Liu ,+ P. Groszewicz, L. Zhao, Dr. Y. Xu, Dr. H. Breitzke, Dr. T. Gutmann, Prof. Dr. G. Buntkowsky Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry Technische Universitt Darmstadt Alarich-Weiss-Strasse 8, 64287 Darmstadt (Germany) E-mail: [email protected] [email protected]
[b] A. Plog Center of Smart Interfaces, Technische Universitt Darmstadt Alarich-Weiss-Strasse 10, 64287 Darmstadt (Germany) [c] Prof. Dr. A. Stark College of Agriculture, Engineering and Science, School of Engineering University of KwaZulu-Natal, Howard College Campus, Durban (South Africa)
spectroscopy and thermogravimetry. CNC-Rh2 catalysts performed well in a model cyclopropanation reaction, in spite of the low dirhodium(II) content on the CNC surface (0.23 mmol g¢1). The immobilization through covalent bonding combined with the separate locations of binding positions and active sites of CNC-Rh2 guarantees a high stability against leaching and allows the recovery and reuse of the catalyst during the cyclopropanation reaction.
milnacipran.[3] Furthermore, they are of interest for enantioand diastereoselective synthesis.[3b] However, unsatisfactory recovery and recycling of dirhodium(II) complexes limit their applications.[1d] Various possibilities for the immobilization of chiral and achiral dirhodium(II) complexes have been discussed previously, such as anchoring the complex in the pores of mesoporous silica[4] or on the side chains of a polymer.[5] Recently, some of us have shown that a dirhodium(II) complex can be grafted on bifunctionalized SBA-15 materials through carboxyl and amino groups, and the structure was determined by 13C and 15N solidstate NMR techniques.[6] In all these works, covalent bonds are achieved by ligand exchange and/or axial coordination.[4, 5f,g, 6] However, for most of these catalysts only moderate activity is observed, probably due to the mass-transfer limitations in porous materials.[7] [d] Dr. R. Hoffmann Eduard-Zintl-Institute for Inorganic Chemistry and Physical Chemistry Technische Universitt Darmstadt Alarich-Weiss-Strasse 12, 64287 Darmstadt (Germany) [e] Prof. Dr. K. Zhang + Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science Technische Universitt Darmstadt Alarich-Weiss-Strasse 8, 64287 Darmstadt (Germany) [f] Prof. Dr. K. Zhang + Wood Technology and Wood Chemistry, Georg-August-Universitt Gçttingen Bìsgenweg 4, 37077 Gçttingen (Germany) E-mail: [email protected] [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501151.
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Full Paper Crystalline nanocelluloses including cellulose nanocrystals (CNC) are a class of biocompatible materials that are formed during the oxidation of microcrystalline cellulose. During the oxidation process carboxyl groups are generated on the surface. These materials have attracted much attention in the last few years,[8] and they have been utilized as templates for chiral structures,[9] as reinforcing components,[10] and recently as supports for photoswitchable molecules such as rhodamine spiroamide.[11] A built-in advantage of CNC is the high specific surface area due to its nanoscale diameter and length.[8b, 12] As proposed by Quignard and Choplin,[13] nanocellulose including CNC has excellent properties as support for catalysts such as CuI,[7b] CuO,[14] Au,[15] AuAg alloy,[16] and Pd nanoparticles.[17] Despite this application potential of CNC as support for nanocatalysts, it has, to the best of our knowledge, not been used for grafting of metal complexes to construct well-recyclable immobilized homogeneous catalysts. Such novel catalyst systems open up new scope in fine-chemical synthesis and the pharmaceutical industry. Using abundant sustainable nanomaterials and minimizing potential heavy-metal pollution is clearly the method of choice for environmentally benign production pathways. Accordingly, the present work describes an efficient way to anchor a dirhodium(II) complex on the surface of CNC through covalent bonds with surface carboxyl groups. The structure of the resulting CNC-Rh2 and details of ligand exchange between the dirhodium(II) precursor and the CNC carrier were determined by solid-state 19F and 13C NMR spectroscopy. The catalytic performance of CNC-Rh2 was investigated in the cyclopropanation of styrene with ethyl diazoacetate (EDA) as a typical model reaction, and the stability against leaching and reutilization of this type of catalyst were studied.
Experimental Section Materials Microcrystalline cellulose (MCC, crystal size of 50 mm), ethyl diazoacetate (EDA), and styrene were purchased from Sigma-Aldrich (Steinheim, Germany). 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) and rhodium trifluoroacetate dimer (Rh2(OOCCF3)4) were obtained from Acros (Geel, Belgium). The dialysis membrane with an approximate molecular weight cutoff of 1000 Da was purchased from Spectrum Laboratories Inc. (Rancho Dominquez, USA). All chemicals were employed without further purification.
Preparation of CNC
Synthesis of CNC-Rh2 The heterogeneous CNC-Rh2 catalyst was obtained by ligand exchange.[6, 19] In a typical procedure, Rh2(OOCCF3)4 (0.1 g, 1.0 equiv) and the acidic form of CNC (CNC-COOH, 1.2 equiv based on the number of carboxyl groups determined by titration, see Supporting Information) in 150 mL of EtOAc was charged into a 250 mL round-bottomed flask. This flask was fitted with a Soxhlet extractor in which an oven-dried mixture of K2CO3 and 4 æ molecular sieves (1 g:1 g) was placed in a cellulose filter tube. To optimize the synthesis, the exchange was performed at 110 8C for 0.5, 2.5, and 4.5 d, respectively. After separation of CNC-Rh2 by centrifugation, the solid was washed three times with both EtOAc and CH2Cl2. The obtained catalysts were stored in CH2Cl2 in a freezer at ¢25 8C. Sample names are abbreviated as CNC-Rh2-0.5d, CNC-Rh2-2.5d, and CNC-Rh2-4.5d, according to the exchange time. A sample containing physisorbed dirhodium(II) complex on CNC (CNC-Rh2-ad) was prepared by simply mixing CNC and Rh2(OOCCF3)4 in EtOAc, followed by centrifugation and washing with EtOAc and CH2Cl2.
Cyclopropanation, leaching, and reusability The cyclopropanation of styrene with EDA was used to evaluate the performance of the heterogeneous CNC-Rh2 catalyst at room temperature.[2c, 4b, 6] Styrene (0.52 g) was transferred to a 25 mL glass bottle containing a solution of 0.035 g of the catalyst in 13.3 g of CH2Cl2, and the mixture was treated with ultrasound for 15 min. The reaction started when EDA (0.057 g in 6.65 g CH2Cl2) was added at room temperature. The reaction mixture was filtered and the clear solution was analyzed by GC (Agilent Technologies 7820A, equipped with an HP-5 (30 m Õ 0.32 mm Õ 0.25 mm) column and FID detector). Leaching was evaluated by a hot filtration test,[20] which was performed simply by centrifugal separation (6000 rpm) at room temperature. Typically, CNC-Rh2-4.5d was removed after 15 min reaction, and CNC-Rh2-ad after 60 min. The recyclability test on CNC-Rh2-4.5d was carried out like the regular catalytic reaction. The catalyst and product were separated by centrifugation (6000 rpm, room temperature) at the end of each reaction run, and the catalyst was reused for the next cycle.
Characterization
CNC was prepared according to literature procedures.[18] Typically, 5 g of MCC was swollen in 50 mL of water for 96 h prior to oxidation. Then, MCC was separated by centrifugation and redispersed in 250 mL of water with vigorous stirring. Thereafter, TEMPO (0.0781 g, 0.5 mmol) and NaBr (0.5144 g, 5 mmol) were added to the cellulose suspension and the mixture was stirred for a further 30 min. An aqueous solution of NaClO (10.89 mL, 14 wt %, 25 mmol) was progressively added to the mixture at pH 10. After addition of the NaClO, the pH value was further maintained at pH 10 by adding 1 m aqueous NaOH. After 15 min of ultrasonic treatment of the reaction mixture at ambient temperature, an additional 10.89 mL of aqueous NaClO (14 wt %) was progressively added to the mixture, and the oxidation was repeated at pH 10. Chem. Eur. J. 2015, 21, 12414 – 12420
After adjustment of the pH value to 7.5 by using 1 m aqueous HCl, the oxidized cellulose was isolated by centrifugation (15 min, 4500 rpm), redispersed in water, and ultrasonicated at room temperature for 1 h. Finally, the suspension was dialyzed until the conductivity reached about 1 mS cm¢1 and its volume was adjusted to 200 mL.
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Solid-state MAS NMR spectroscopy: Solid-state 19F MAS NMR spectra were recorded at 564.2 MHz on a Bruker Avance III spectrometer with a broadband double-resonance 1.3 mm probe. All spectra were typically accumulated with a recycle delay of 3 s and a spinning rate of 60 kHz. The 19F chemical shifts were referenced to trifluoroacetic acid at ¢76.5 ppm. The fluorine content was determined by quantitative 19F MAS NMR (see Supporting Information for details). 13C CP MAS NMR was carried out with a broadband double-resonance 4 mm probe at 150.81 MHz. 13C CP MAS spectra were recorded with a recycle delay of 3 s, 3 ms contact time, and a spinning rate of 6 kHz. The 13C chemical shifts were referenced to TMS at 0 ppm.
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Full Paper Thermogravimetry (TG): TG measurements were carried out with TG209F1-Iris from Netzsch (Selb, Germany). Each sample was analyzed under an oxygen atmosphere in the range of 30–800 8C with a heating rate of 10 8C min¢1 on an alumina crucible. Atomic force microscopy: AFM images were obtained on a scanning force microscope (MFP-3D, Asylum research, Santa Barbara, USA) with a cantilever (Budget Sensors, Sofia, Bulgaria, spring constant kc 48 N m¢1, resonance frequency 190 kHz) in tapping mode. The samples were deposited from suspensions on a silicon wafer and dried overnight. The programs IGOR Pro 6.22A (WaveMetrics Inc., Lake Portland, USA) and Gwyddion (Free Software Foundation, Inc., Boston, USA) were used to guide and analyze the AFM measurements, respectively.
Results and Discussion Immobilization of the dirhodium(II) complex on CNC CNC with carboxyl groups on the surface was prepared by TEMPO-mediated oxidation of microcrystalline cellulose (Figure 1 a).[17, 20] During oxidation, the amorphous regions inside the cellulose fibers are cleaved, which leads to the formation of CNC. The primary hydroxyl groups at the C6 position on the surface of cellulose fibers are transformed first into aldehyde groups and further into carboxyl groups during the oxidation
(Figure 1 a).[18] The contents of carboxyl and aldehyde groups were determined to be 1.101 and 0.053 mmol g¢1, respectively, by conductive titration (Supporting Information, Figure S1).[12, 22] Similar to the literature,[23] the resulting CNC exhibits a diameter of approximately 6 nm and a length of around 200 nm according to AFM measurements (Figure 1 b–d). In the next step, the dirhodium(II) moieties were anchored on the surface of CNC by ligand exchange between Rh2(OOCCF3)4 and carboxyl groups (Figure 1 e). The trifluoroacetic acid byproduct of the exchange reaction was evaporated, condensed, and then neutralized with K2CO3 during Soxhlet extraction.[19] The resulting CNC-Rh2 catalyst still maintained the dimensions of CNC with an average diameter of about 8 nm and a length of about 200 nm (Figure 1 f–h). The presence of the rhodium was confirmed by energy-dispersive X-ray spectroscopy (EDX) analysis (Supporting Information, Figure S2), and the amount of dirhodium(II) moieties on CNC was determined by TG (Figure 2). According to the literature, Rh2O3 is formed when dirhodium(II) species are heated at 500 8C in TG analysis under an atmosphere of air[24] and is stable up to 1100 8C.[25] Therefore, the amount of dirhodium(II) moieties in each sample was calculated from the amount of residue at 800 8C after TG analysis. The fraction of dirhodium(II) units in CNC-Rh2-4.5d was estimated to be 0.23 mmol Rh2 per gram of the material. For CNC-Rh2-0.5d and CNC-Rh2-2.5d values of 0.19
Figure 1. a) Schematic illustration of the synthesis of CNC by TEMPO-mediated oxidation. b), c) Representative AFM images of CNCs at different magnifications: scale bar of 500 nm in (b) and 200 nm in (c). d) Height profile of the CNC in (c). e) Schematic illustration of the preparation of CNC-Rh2. f), g) Representative AFM images of CNC-Rh2-4.5d at different magnifications: scale bar of 500 nm in (f) and 200 nm in (g). h) Height profile of the CNC in (g). Chem. Eur. J. 2015, 21, 12414 – 12420
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Figure 2. TG curves and amount of residue after destruction in oxygen atmosphere (30–800 8C) for pristine CNC (d) and CNC-Rh2-4.5d (c).
Figure 3. Deconvoluted solid-state 19F MAS NMR spectra of CNC-Rh2-4.5d (a) and the physical mixture of CNC and neat Rh2(OOCCF3)4 (b).
and 0.22 mmol Rh2 per gram of the material were found (Supporting Information, Figure S3 a), and for the sample CNC-Rh2ad containing physisorbed catalyst the amount of Rh2 was below the detection minimum of the TG analysis. (Supporting Information, Figure S3 b) Since the preparation of heterogeneous CNC-Rh2 was carried out in analogy to the route for generation of homogeneous dirhodium(II) complexes,[19] it was assumed that the replacement of each carboxylate group in the dirhodium(II) complex occurs stepwise, as shown by kinetic studies in the literature.[26] Additionally, Doyle et al.[2c] pointed out that pure Rh2(HNOCCH3)4 is only obtained from Rh2(OOCCH3)4 by trapping the acetic acid, that is, removal of this byproduct can drive the exchange reaction to completion. In our work, the ligand-exchange reaction takes place on the surface of CNC, and the gradual removal of trifluoroacetic acid indeed promoted the ligand exchange. 19
F Solid-state NMR characterization
Since the ligand-exchange process occurs stepwise,[26] the remaining trifluoroacetate on the CNC-Rh2 may provide information on the dirhodium(II) units and the structure of this catalyst after ligand exchange. Thus, solid-state 19F MAS NMR measurements were performed. The signals in the 19F spectrum of CNC-Rh2-4.5d (Figure 3 a), which are located in the region of those of neat Rh2(OOCCF3)4, indicate the presence of the dirhodium(II) units bearing trifluoroacetate on CNC. The spectrum of CNC-Rh2-4.5d shows a main peak at about ¢76 ppm, which is related to the presence of trifluoroacetate ligands coordinated to dirhodium(II) units on the surface of CNC. In addition, a less intense component shifted by ¢3 ppm from the main peak is observed. This signal seems to be attributable to the proximity of water molecules, which influence the chemical environment of the trifluoroacetate ligand and thus the 19F chemical shift. This assumption is based on the observed influence of humidity on the solid-state 19F NMR spectrum of the neat Rh2(OOCCF3)4 complex (see Supporting Information, Figure S4). To support this hypothesis, a dry physical mixture of CNC and neat Rh2(OOCCF3)4 (1.5 wt %, ca. 0.23 mmol g¢1) was invesChem. Eur. J. 2015, 21, 12414 – 12420
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tigated. Only one peak at ca. ¢76 ppm was observed in the 19F spectrum (Figure 3 b), which implies that the extra peak in the immobilized sample is attributable to the presence of water, which is formed by neutralization of the trapped CF3COOH with K2CO3 during catalyst preparation. Furthermore, the amount of fluorine on CNC was determined quantitatively by 19F NMR spectroscopy and was found to be 1.32 0.23 mmol g¢1 for CNC-Rh2-4.5d (for details, see Supporting Information). Taking the Rh2 fraction (0.23 mmol g¢1) on CNC-Rh2-4.5d into consideration, the CF3COO¢/Rh2 ratio is 1.91 0.33, which is close to a factor of two. This means that, on average, two CF3COO¢ groups remained in the catalyst after ligand exchange. This observation is in good agreement with a kinetic study on ligand exchange between trifluoroacetic acid and Rh2(OOCCH3)4.[26] It showed that 1) ligand exchange proceeded stepwise, 2) the second substitution step occurred trans to the first one, and 3) the third and fourth substitution had much lower rate constants than the first two steps; this led to the assumption of transsubstituted CNC-Rh2 (Figure 1 e). 13
C Solid-state NMR characterization
In Figure 4, solid-state 13C CP MAS NMR spectroscopy was employed to analyze the chemical environment of CNC-Rh2 in comparison with pristine CNC, especially for carboxyl groups in the neutral (CNC ~ COONa) or the acidic form (CNC ~ COOH). The spectra in Figure 4 b show strong signals that belong to the cellulose domain (50–110 ppm) and carboxyl carbon atoms (165–190 ppm). The signals at 104 and 65 ppm are attributed to C1 and C6 in the anhydroglucose units of cellulose, respectively.[11, 27] The signals between 74 and 72 ppm are ascribed to C3/C2/C5, and the signals at 88 and 84 ppm are related to crystalline and amorphous C4, respectively.[28] The carbonyl regions of the spectra are similar to those previously reported for the CNC–rhodamine system.[11] For CNCs with carboxyl groups in their acidic (CNC ~ COOH) and salt (CNC ~ COONa) forms, signals at 171 and 174 ppm, respectively, are observed. A signal at 174 ppm is also observed in the spectrum of the
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Figure 5. a) Schematic representation of the catalytic model cyclopropanation reaction.[4b] b) Catalytic performance of the heterogeneous CNC-Rh24.5d catalyst: total yield and percentages of the cis and trans isomers as functions of the reaction time. c) Leaching test: reaction in the presence of CNC-Rh2-4.5d (1) and after removal of CNC-Rh2-4.5d (1’), as well as in the presence of CNC-Rh2-ad (2) and after removal of CNC-Rh2-ad (2’).
Figure 4. a) Schematic illustration of the synthesis of CNC in its COOH and COONa forms. b) Solid-state 13C CP MAS NMR spectra of CNC ~ COOH (top), CNC ~ COONa (middle), and CNC-Rh2-4.5d (bottom). Signals marked with asterisks are spinning side bands.
CNC-Rh2-4.5d sample, which clearly indicates CNC ~ COONa. In addition, a very weak signal with low signal-to-noise ratio around 188 ppm is observed. Similar chemical shifts were observed for the carboxyl carbon atoms of the linkers in the supported Rh2(OOCCH3)4.[6] Thus, this signal is an indication of carboxyl groups coordinated to rhodium on the surface of the CNC. Activity tests The cyclopropanation of styrene with EDA was employed as model reaction to investigate the performance of the heterogeneous CNC-Rh2 catalyst (Figure 5 a). To determine synthesis conditions under which the heterogeneous CNC-Rh2 catalyst performs best, the catalytic reaction was performed with equal amounts of the CNC-Rh2-0.5d, CNC-Rh2-2.5d, and CNC-Rh2-4.5d immobilized catalysts (Supporting Information, Figure S6). From the GC results, the trans isomer is slightly favored for most of the heterogeneous catalysts, but the cis:trans ratio is always close to 1:1 (Supporting Information, Figure S8). This is similar to the results observed for cyclopropanation of styrene with homogeneous Rh2(OOCCF3)4.[2c, 29] CNC-Rh2-4.5d (Figure 5 b) exhibited the highest yield of about 75–80 % in 180 min at room temperature. For comparison, CNC-Rh2-ad with physisorbed Rh2(OOCCF3)4 on the CNC surface was investigated (Figure 5 c). It exhibited the lowest yield of only 7 %, which is not surprising, because the amount of rhodium in the sample is below the detection minimum of the TG analysis (see Supporting Information, Figure S3 b). This result demonstrates that the contribution of physisorbed Rh2(OOCCF3)4 on CNC to the overall yield is negligible in comparison to its covalently immobilized counterpart. Chem. Eur. J. 2015, 21, 12414 – 12420
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The catalytic activities at room temperature of the new heterogeneous catalyst and its homogeneous counterpart Rh2(OOCCF3)4 were compared. The concentration of the homogeneous catalyst was chosen according to the Rh content of CNC-Rh2-4.5d. The homogeneous catalyst reached a yield of 84 % in 1 min, which then increased to 95 % after 1 h (see Supporting Information, Figure S7 a). The initial rate constants for the heterogeneous and homogeneous catalysts were approximated by assuming a first-order reaction. From these fits (see Supporting Information, Figure S7), 0.044 min¢1 was calculated for the heterogeneous catalyst, as opposed to 2.13 min¢1 for the homogeneous catalyst. This shows that, although the immobilized catalyst exhibits excellent performance in the catalytic cyclopropanation under mild conditions, its activity seems to be limited by mass transfer and loss of mobility, because the catalytic centers are confined on the surface of the CNC support, and transport processes are the rate-determining factor. Leaching test In the leaching test performed at room temperature on CNCRh2-ad (Figure 5 c), the yield of product in the filtrate solution stayed nearly constant following removal of CNC-Rh2-ad after 60 min, which indicates that the small amount of physisorbed catalyst is stable against leaching and thus the catalytic activity in this sample stems from catalyst molecules that interact with the CNC. We assume that this robustness against leaching results from the factor that CNC stabilizes the Rh2(OOCCF3)4 on its surface through physical interactions. In the case of CNC-Rh2-4.5d, the reaction did not take place after the catalyst was removed by centrifugation for 15 min (Figure 5 c), which confirms that significant leaching of active sites does not occur. According to the results of solid-state NMR spectroscopy, the covalent bonds are formed between the carboxyl group of CNC and dirhodium units, whereby the carboxyl groups coordinate with dirhodium moieties at equa-
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Full Paper torial positions (see Figure 1 e). The catalytic sites of the dirhodium catalyst are at the axial position, where a rhodium carbenoid is formed (see Supporting Information, Scheme S1).[1d, 2b] Therefore, the separation of catalytic sites (axial direction) and binding sites (equatorial position) seems to prevent release of the dirhodium(II) moieties from the CNC surface. The recyclability of CNC-Rh2-4.5d was investigated under similar conditions to the catalytic test (Figure 6). The obtained yields in the first three runs stayed almost at the same level, with a slight decrease of only 5 % demonstrating its excellent recyclability in the cyclopropanation reaction.
mogeneous catalysts, employing dirhodium(II) as a real-world example. The potential application in constructing sustainable immobilized catalysts is not limited to dirhodium(II), and various candidates seem to be feasible for grafting on the CNC surface by using our methodology for heterogenization of homogenous catalysts.
Acknowledgements The authors thank the Hessian excellence initiative LOEWE – research cluster SOFT CONTROL and Deutsche Forschungsgemeinschaft (DFG) under contract Bu 911-12-2 for the financial support. Keywords: cellulose · cyclization · nanoparticles · rhodium · supported catalysts
Figure 6. Recyclability test of CNC-Rh2-4.5d in the cyclopropanation between styrene and EDA. The yield reached after each run is plotted.
Conclusion A heterogeneous CNC-based dirhodium(II) catalyst was prepared by ligand exchange with sustainable cellulose nanocrystals as biocompatible support. This heterogeneous catalyst exhibits good catalytic performance even at low dirhodium(II) loadings (0.23 mmol g¢1). The structure of this catalyst was investigated by solid-state NMR spectroscopy. 13C CP MAS NMR spectra confirmed a chemical bond between the dirhodium(II) moiety and carboxyl groups on CNC, and 19F MAS NMR spectroscopy indicated the existence of trifluoroacetate groups on CNC-Rh2. On the basis of quantitative 19F NMR and TG analysis, an average of two trifluoroacetate ligands are replaced by COOH groups on the CNC surface. The content of the dirhodium(II) units on the CNC carrier increases to some extent at longer exchange reaction times. CNC-Rh2-4.5d shows the highest concentration of catalytic sites among the prepared catalysts. Its superior performance in the model cyclopropanation reaction indicates that the majority of the Rh sites are available and play an active role in the reaction. The leaching test showed no release of catalytic sites from the CNC support during the cyclopropanation reaction, and the recyclability test revealed excellent recovery and reusability of this heterogenized catalyst. The lower rate constant obtained for the immobilized catalyst compared to the homogeneous dirhodium(II) catalyst is typical for a change from homogeneous to heterogeneous catalysis, for which transport properties are often the ratedetermining factor. However, the lower overall rate constants are a small trade-off for its ease of separation from the reaction medium, its recyclability, and its possible application in continuous reaction processes. Thus, we have shown that CNCs are an excellent support material for the heterogenization of hoChem. Eur. J. 2015, 21, 12414 – 12420
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Received: March 24, 2015 Published online on July 14, 2015
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